Turbofan gas turbine engines (which may be referred to simply as ‘turbofans’) are typically employed to power aircraft. Turbofans are particularly useful on commercial aircraft where fuel consumption is a primary concern. Typically a turbofan gas turbine engine will comprise an axial fan driven by an engine core. The engine core is generally made up of one or more turbines which drive respective compressors via coaxial shafts. The fan is usually driven directly off an additional lower pressure turbine in the engine core.
The fan comprises an array of radially extending fan blades mounted on a rotor and will usually provide, in current high bypass gas turbine engines, around seventy-five percent of the overall thrust generated by the gas turbine engine. The remaining portion of air from the fan is ingested by the engine core and is further compressed, combusted, accelerated and exhausted through a nozzle. The engine core exhaust mixes with the remaining portion of relatively high-volume, low-velocity air bypassing the engine core through a bypass duct.
The rotor of the fan can be considered to be a hub. The fan blades are connected to the hub via a blade root. The hub generally includes a plurality of axially extending grooves in the periphery of the hub to receive the roots of the fan blades. To axially retain the fan blades in the hub a shear key and shear key slot arrangement is used. The design of the shear key and shear key slot arrangement can improve accuracy of positioning of the fan blades into the hub and contribute to preventing a fan blade being released from the hub in the event of the blade being impacted, e.g. by a foreign object such as a bird.
An exemplary shear key 172 and shear key slot 160 is shown in FIGS. 1A and 1B. The key locates in the shear key slot provided in the blade root 152. One slot is located each side of the root so that each slot portion receives one arm of the key. An undercut 162 is provided in the base of the root portion to receive the bridging portion (or joining member) 176 of the key which interconnects its arms 174. The groove in the hub which receives the fan blade root is also provided with two generally radially extending slots. The axial extent of each of the slots in the groove of the hub is approximately equal to the thickness of the arms of the key. When the fan blade root portion is correctly positioned within the fan hub, the slots in the blade root and hub groove respectively are radially aligned. This permits the arm of the key to simultaneously locate in the slots in the blade root and the slots in the groove of the hub. As a consequence of this, the fan blade root portion is prevented by the key from translating axially relative to the hub.
As can be seen from FIG. 1A, the shear key slot 160 has two flat faces extending in a thickness direction of the blade, the two flat faces are linked by a further flat face extending substantially in the chordwise direction of the blade (if the blade root is curved the flat face intersects two points on a chord line of the root). The corners between the planar faces are curved to reduce the stress concentration. The circumferentially extending face is provided so that the shear key slot arrangement is capable of resisting bird strike impact forces.
Typically the surface of the shear key slot 160 and key 172 is treated to mitigate fatigue failure. An exemplary surface treatment method is laser shock peening. Laser shock peening is an expensive treatment method. Furthermore, care needs to be taken that the corners between the planar faces of the key slot are correctly treated, because the corners are geometrically complicated to treat but are also generally the region of highest stress concentration.